Extracting the shape of a vascular network is one of the most common tasks in medical image processing, and there is considerable literature on the subject. Photoshop isn't a good choice for scientific or medical image processing. PS is more for graphic designers, photographers, artists, etc., not scientists and physicians. You would be much better served with Image J from NIH. The image processing toolkits for Matlab and Mathematica would be distant second choices, but still far ahead of PS because PS simply does not have the many of the most common primitives needed for such processing, e.g., morphological operators, image regularization, a good interface to mathematical operations, etc., whereas these are exactly the areas in which ImageJ, Matlab and Mathematica shine.
For example, many ImageJ plugins / add-ons have been written for angiogenesis and similar analyses:
Vascular Network Toolkit for ImageJ:
http://sourceforge.net/projects/ntwrkanlystlkit/
http://ntwrkanlystlkit.sourceforge.net/overview.php
Angiogenesis Analyzer for ImageJ:
http://image.bio.methods.free.fr/ImageJ/?Angiogenesis-Analyzer-for-ImageJ
A Computational Tool for Quantitative Analysis of Vascular Networks:
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0027385
HTH,
Tom M
PS - I presume the images you are concerned with are all taken without fluorescing or other chemical contrast agents, and that for your application, these can't be used, right?